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PENDULUM TESTS USING RIGID AND CRUSHABLE BUMPERS

The test program discussed in this paper consisted of 19 tests of a breakaway sign support using a 2,000-lb (907-kb) pendulum mass impacting the support at 20 mph (32 km/h). Use of both rigid and crushable bumpers permitted examination of these techniques compared with current momentum change criterion. In addition, the effects of bolt-tightening torque on the slip-base release loads were investigated. Hi-Lok frangible nuts, which control tightening torque, were also evaluated for the design torque condition. Only in the crushable bumper tests were dramatically different results obtained for the various nut-tightening torques when momentum change was used as the criterion. Momentum change with the hard bumper was 65 plus minus 15 lbf-sec (289 plus minus 67 N-s) for all base-nut torque levels. Momentum change with the crushable bumper ranged from 88 lbf-sec (391 N-s) for design torque condition to 398 lbf-sec (1770 N-s) for the overtorqued condition. Repeatibility of slip-base loads was generally good when both a calibrated torque wrench and the Hi-Lok torque control nuts were used. It was concluded that momentum-change criterion is insufficient in evaluating results of pendulum tests using a rigid bumper. Use of a bumper with vehicle crush characteristics appears to provide a superior experimental evaluation. /Author/